In recent years, social request to energy saving is allowing requirement to a reduction in fuel consumption of cars to be severer. In order to meet such requirement, a rolling resistance in tire performances is required to be reduced. A method for reducing a rolling resistance of a tire by optimizing a tire structure has been investigated, but it is carried out as the most usual method to use a material having a low heat buildup property as a rubber composition.
In order to obtain such a rubber composition as having a low heat buildup property, a large number of techniques for enhancing a dispersibility of a filler used for a rubber composition has so far been developed. Among them, particularly a method in which a polymerizable active site of a diene base polymer obtained by anionic polymerization using a lithium compound is modified with a functional group having interaction with a filler is getting most popular.
Known as the most representative method among such methods are a method in which carbon black is used as a filler and in which a polymerizable active site is modified with a tin compound (Japanese Patent Publication No. 87630/1993) and a method in which carbon black is used in the same manner and in which an amino group is introduced into a polymerizable active end (Japanese Patent Application Laid-Open No. 207342/1987).
On the other hand, in recent years, requirements not only to a low fuel consumption performance but also to a performance on a wet road surface (hereinafter referred to as a wet performance), particularly a braking performance has increased as concern about a safety of cars grows high. Accordingly, not only a mere reduction in a rolling resistance but also a high compatibility of a wet performance with a low fuel consumption performance are required to the performances of a rubber composition in a tire tread.
A method using silica in place of carbon black which has so far usually been used as a reinforcing filler has already been carried out as a method for obtaining a rubber composition providing a tire with such a good low fuel consumption and a good wet performance at the same time.
However, it has been apparent that when using silica as a reinforcing filler, a fracture strength and an abrasion resistance characteristic of a rubber composition can not be avoided from being notably reduced as compared with carbon black. Further, silica is inferior in dispersibility, and the workability in kneading has been a large problem in actually producing tires.
Accordingly, in order to obtain a rubber composition having a good thermal property, not only carbon black or silica is used alone, but also carbon black and silica are used in combination, and in addition thereto, required is an active site-modified polymer which has a wide interaction with a variety of such fillers and which can provide the fillers with a good dispersibility and the rubber composition with an abrasion resistance.
However, it is the existing situation that since an active site-modified polymer has so far been developed assuming that a single filler is used, active site-modified polymers having a satisfactory interaction with various fillers regardless of the kind of the fillers are restricted very much.
For example, the tin compound described above has a large dispersing effect against carbon black but little dispersing effect against silica, and in addition thereto, it does not exhibit a reinforcing effect at all. Further, a dispersing effect of aminosilane against silica is reported in Japanese Patent Application Laid-Open No. 151275/1997, but the effects thereof are not necessarily satisfactory.
On the other hand, disclosed is a method using alkoxysilane providing silica with a dispersing effect and an effect for improving a reinforcing property (Japanese Patent Application Laid-Open No. 188501/1989, Japanese Patent Application Laid-Open No. 53513/1996 and Japanese Patent Application Laid-Open No. 53576/1996). However, it is apparent that since an alkoxysilyl group does not have at all interaction with carbon black, no effects are expected when carbon black is used as a reinforcing filler. Also, the same shall apply to other active site-modified polymers, and a method using, for example, aminoacrylamide is disclosed (Japanese Patent Application Laid-Open No. 71687/1997 and Japanese Patent Application Laid-Open No. 208633/1997). However, while this method has an effect for improving a dispersibility of silica to some extent, an effect for improving a dispersibility of carbon black is scarcely observed, and the problem that the hysteresis loss goes up is observed in the cases of a rubber composition of a system in which carbon black and silica are used in combination and a rubber composition in which carbon black is blended.
Further, disclosed is a modified polymer obtained by introducing alkoxysilane having a dialkylamino group into an active end of a polymer obtained by anionic polymerization using alkyllithium or lithiumamide as a polymerization initiator (Japanese Patent Publication No. 53763/1994 and Japanese Patent Publication No. 57767/1994). When using this modified polymer, obtained are a reinforcing property corresponding to silica blended and a fixed dispersing effect to both silica and carbon black as well as a good workability, but because of an amino group which is a dialkyl group-substituted type having less effect to carbon black, particularly a blend system containing a large amount of carbon black is not provided with a satisfactory effect as compared with a case where a modified polymer obtained by using a tin base modifying agent is used.
On the other hand, a polymer having an active site used in producing a modified polymer is obtained usually by anionically polymerizing a conjugated diene compound alone or a conjugated diene compound with an aromatic vinyl compound. It is not easy in terms of characteristics in anionic polymerization to introduce a functional group such as a primary amino group and an organic onium base which are expected to be very effective for improving the physical properties into an active site of the above polymer obtained by anionic polymerization. Post polymerization treatment under severe conditions and an expensive protective group are required, and therefore the industrial value thereof is low.
In these methods, one functional group at most is introduced into an active site of a polymer, and such complicated synthetic techniques as using a dilithium base initiator and a macromonomer have to be used in order to introduce a plurality of the functional groups described above per molecule of the polymer. Accordingly, they are not methods which are liable to be industrially applied. Further, in producing a rubber composition, the physical properties are tried to be improved by subjecting the functional groups described above to heat mechanical treatment with a hydrocarbyl compound and a silane-modified polymer. In this case, the effects thereof are unsatisfactory, and in addition thereto, brought about are the problems that a valuable machine time of a kneading machine is spent and that an evaporating amount of alcohol in the kneading machine grows large. Thus, they are not methods which are industrially preferred.
Under such circumstances, an object of the present invention is to provide a process for producing a modified polymer which enhances interaction between silica and carbon black when used for both of silica-blended and carbon black-blended rubber compositions and which elevates a fracture characteristic, an abrasion resistance and a low heat buildup property at the same time and can exhibit a good workability, a modified polymer having the characteristics described above, which is obtained by the process described above, a rubber composition comprising the above modified polymer and a pneumatic tire prepared by using the above rubber composition.